文档细节优化，search、咯阿丁和dialog组件样式修复 (#5)

* fix: loading small style, search style and dialog style

* fix: scroll to top

* fix mobile scroll

* fix scroll to top

packages/vant-css/src/actionsheet.css

@@ -21,16 +21,13 @@
 
     @e item {
       line-height: 50px;
+      height: 50px;
       text-align: center;
       color: $c-black;
       font-size: 16px;
       position: relative;
       background-color: $c-white;
 
-      @m loading {
-        padding: 10px 0;
-      }
-
       &::after {
         @mixin border-retina (top);
       }
@@ -43,6 +40,7 @@
 
     @e loading {
       margin: 0 auto;
+      display: inline-block;
     }
 
     @e button {

packages/vant-css/src/dialog.css

@@ -71,8 +71,9 @@
 
     @e btn {
       font-size: 16px;
-      line-height: 40px;
+      line-height: 52px;
       border: 0;
+      padding: 0;
       background-color: #fff;
       float: left;
       box-sizing: border-box;

packages/vant-css/src/loading.css

@@ -24,10 +24,20 @@
 
 @component-namespace van {
   @b loading {
-    width: 30px;
-    height: 30px;
+    
     line-height: 0;
     font-size: 0;
+
+    @m circle {
+      width: 16px;
+      height: 16px;
+      vertical-align: middle;
+    }
+
+    @m gradient-circle {
+      width: 30px;
+      height: 30px;
+    }
   
     @e spinner { 
       display: inline-block;

packages/vant-css/src/search.css

@@ -28,6 +28,7 @@
       color: $c-gray-dark;
       border: none;
       outline: none;
+      -webkit-tap-highlight-color: rgba(0, 0, 0, 0);
     }
 
     @e cancel {
@@ -51,10 +52,11 @@
 
     .van-icon-clear {
       font-size: 14px;
-      line-height: 1;
+      line-height: 16px;
       position: absolute;
       right: 5px;
-      top: 9px;
+      top: 50%;
+      transform: translateY(-50%);
       color: #888;
     }
   }

packages/vant-css/src/switch.css

@@ -37,6 +37,7 @@
       left: 50%;
       top: 50%;
       transform: translate3d(-50%, -50%, 0);
+      margin: 0;
     }
 
     @m on {